1. Field of the Invention
The present invention generally relates to the processing of polymer compounds and mixtures in a mixer of the closed type.
This kind of processing is commonly performed in the manufacture of tyres or components thereof such as tread bands and the like; such processing consists of mixing a polymer base together with other ingredients according to predetermined recipes, in order to obtain semi-finished products to be used in the subsequent phases of the manufacturing cycle.
2. Description of the Related Art
Closed mixers, also known as discontinuous mixers, are substantially formed by a mixing chamber accommodating a pair of rotors rotating in mutually opposite directions, wherein the material is loaded in predefined quantities (batches) and processed by the rotors which completely discharge it from the mixer at the end of the cycle, before beginning to process a new batch of material.
In this connection it should be considered that the term “mixture” in this description is intended to indicated the generic intermediary product obtained during any phase of the processing cycle, while the term “compound” will be used to refer to the semi-finished product derived from the addition of the cross-linking system to the mixture in order to prepare it for subsequent vulcanization.
The chemical-physical characteristics of the mixtures and the compounds, which allow an end product with desired qualities to be obtained, depend also on the processing in the closed mixers; for this reason the control of this operating phase has a decisive role in the manufacture of tyres and their components.
The process variables which influence the working process in closed mixers are many and it is therefore difficult to ensure an adequate control thereof throughout the whole operating cycle, i.e. from the filling of the mixer to its emptying.
Examples of these process variables are the characteristics of the ingredients used for compounds and mixtures, their temperature, the speed and the torque applied to the rotors, the filling level of the mixer (i.e. the so-called “fill factor”) and others.
Nowadays various mixing control systems are known, which are based on the detection over time of certain process parameters (also called “indirect” parameters), such as the temperature of the mixtures or the energy conferred to the system, and on the eventual correction thereof by modifying other parameters (also called “direct” parameters) which directly determine the functioning of the mixer, such as, for example, the speed of the rotors, the pressure of the pressing ram and so on.
Examples of this procedure are described in international patent application No. PCT/US98/23294 and in U.S. Pat. No. 4,830,506.
In particular the latter describes a system for controlling processing in a closed mixer, based on regulating the pressure of the pressing ram so as to ensure that when the ram is in the lowered operating position, it can oscillate with a small predetermined amplitude (in the order of half an inch, i.e. approximately 12.5 mm).
According to this US patent, with such a control system it is possible to obtain an improved mixing action by making use of the periodic rebounds due to the rotation of the rotors.
One characteristic of compounds and mixtures which, until nowadays, closed mixers and their regulating systems did not control efficiently, is the dispersion of the various ingredients within the processed mass.
This characteristic is of considerable importance for obtaining an end product with the required qualities, both because a non-homogenous dispersion of an ingredient in the mixed material can cause local defects in the tyre or semi-finished product which will be manufactured, and because a different dispersion between one charge of material processed in the mixer and the subsequent one, can result in lack of uniformity between “batches” with the same basic formulation and can negatively influence the repeatability of the rheometric curve of the compound.